Cytoplasmic Ca transients trigger key cellular events such as contraction and secretion. The organelle specifically devoted to the rapid uptake of Ca and to its release in response to receptor- mediated generation of inositol 1,4,5-trisphosphate (IP3) has been suggested to be the endoplasmic reticulum (ER). We have found instead that the IP3-sensitive Ca store seems to correspond to a newly recognized organelle which we term "calciosome". This organelle consists of small vesicles and irregular vacuoles which are not continuous with the ER or Golgi complex and are distinct from mitochondria, lysosomes and secretory granules. The calciosome is endowed with a Ca-pump, an IP3-gated Ca channel and an intraluminal Ca binding protein similar to muscle calsequestrin (CS). The long-term objective of the proposed research is the biochemical, functional and morphological characterization of the calciosomes of exocrine pancreas and brain. The purification of the organelle and its biochemical characterization will involve measurements of Ca fluxes, and enzymatic, immunological and binding assays. Two key calciosome proteins (the CS-like protein and the IP3-binding sites) will be isolated, characterized and used for eliciting antibodies. As to functional characterization, kinetics and regulation of IP3-induced Ca release will be studied, as well as monovalent cation, anion and proton permeability pathways in purified calciosomes. These studies will involve measurements of Ca fluxes by isotope and spectrophotometric methods and patch clamping. Morphological characterization will involve immunolabeling of ultrathin cryosections to assess the subcellular distribution of the calciosome, in both quiescent and activated cells, and its relationship to other known organelles. The results of this investigation will have health-related implications for the nervous and exocrine systems, which rely on IP3-induced CA release for several key functions. Similarly, the results obtained here may be applicable to the cardiovascular system, where IP3 is known to play a key role in smooth muscle contractility and in platelet activation. Identification of Ca release modulators may also bring about new pharmacologic approaches for specific diseases.